This invention relates to an incrementally adjustable distributed network structure, and it relates, more particularly to such a network arrangement employing a plurality of delays and a number of controlled sources interconnected in distributive fashion.
In microwave circuits for point-to-point, also line-of-sight, communication and radar technology, it is often necessary to adjust the transmit and receive channels to an exact specified amplification, or to change the amplification electronically over many decibels (e.g. 20 dB). This requirement comes about, particularly with phased array radar equipment, where, in addition to phase gradation, amplitude gradation (weighting) of the individual channels is often employed to optimize the radiation characteristic.
Mechanically switchable networks, including attenuating elements, are usually employed for the step-wise or incremental adjustment of the amplification and/or attenuation of high frequency microwave channels. If the switching is carried out electronically, the related switching diodes and transistors introduce additional, unavoidable, often undesirable insertion loss. This additional attenuation adversely affects adjustment range and maximum amplification.
If, in addition, the maximum possible attenuation range is to be attained with the minimum possible number of attenuating elements, the steps or increments in attenuation produced by the individual elements must be graded or weighted to correspond with a geometrical series with q=2. In that case, greater accuracy and tolerance requirements are imposed on the elements producing higher attenuation, or discontinuities in the attenuating steps will occur under some conditions. These types of networks are therefore relatively tolerance sensitive; for example, in the case of a 1 dB increments of attenuation with binary weighted series elements (1, 2, 4, 8. . . dB) the 8 dB series element must exhibit a negative error of less than 6.5%, if the 1 dB, 2 dB and 4 dB network elements have an equally large positive error of 6.5%. When these tolerances are exceeded, the desired increase in attenuation does not result in this case, but instead an (unallowable) reduction in attenuation occurs, during transition of the network from the 1 dB-, 2 dB-, and 4 dB series elements to the 8 dB series element. The higher the attenuation produced by an individual series element, the more stringent the tolerance requirement becomes relative to the attenuation of this individual element. If, for example, in the gradation just described, series elements with 1, 2, 4, 8, 16 dB are employed, the 16 dB series element may only exhibit a negative error of less than 3.17%. The series connection of attenuating elements in networks for the amplitude gradation of microwave signals is therefore a technically demanding method of fabrication.
Either the step-wise insertion of networks of attenuating elements in the amplifier train or the use of amplifiers with step-wise variable gain may be employed for the step-wise variation of channel amplification. Should the adjustment range encompass many decibels and apply, with close tolerances, over a wide frequency range, e.g. over a frequency range between one half and one octave, then the control characteristics of transistors are too inaccurate and the circuits too narrow banded for this purpose. Space and cost considerations rule out the use of mechanical switching for the adjustment of gain in many areas of microwave technology.